Inductive proximity sensors can be employed to provide non-contact detection of an object or target. Such sensors can be utilized for a variety of sensing functions in connection with industrial plants and/or machinery. For instance, inductive proximity sensors can be employed in connection with material handling systems, robot systems, assembly systems and machines, etc. An inductive proximity sensor can emit an output signal when a target enters into a sensing area. Typically, the target is a metallic object such as a ferrous material (e.g., iron, steel, etc.) or other metallic materials (e.g., copper, nickel chromium, brass, aluminum, etc.).
A typical inductive proximity sensor operates by generating a magnetic field from a detection face. When a target moves into the magnetic field, eddy currents swell in the target. The eddy currents, in turn, generate a magnetic field, which interacts with the magnetic field generated by the sensor. In particular, the magnetic field generated by the eddy currents operates to dampen the magnetic field generated by the sensor. The sensor detects the dampening of the magnetic field and triggers and output signal which indicates that the target is in proximity to the sensor location.
Inductive proximity sensors generate stray fields or magnetic flux out the back, the sides, and/or any other direction away from a sensing face of the sensor. In some cases, stray fields can be reduced, but not eliminated, with shielding. The stray field renders the inductive proximity sensors sensitive to metal behind and/or to the sides of the sensors, including mounting means. Interactions with ancillary metal reduce sensitivity and sensing distance of inductive proximity sensors. One way to combat loss of sensitivity is to engineer sensors, which are to be mounted in a particular manner. However, even when mounted according to the intended configuration, surrounding metal continues to affect the sensor and ultimately impacts a maximum achievable sensitivity and temperature stability.
Further, two of the largest sources of undesired damping in unmounted sensors are resistance of the coil winding and damping due to metal housing of the sensor. Numerous techniques have been devised to ameliorate effects of coil resistance, but little has been devised in regard to reducing damping due to housing.
Housings are commonly composed of inexpensive brass, with a resistance ranging from about 7 to 15 times that of copper, depending on a percentage of zinc in the brass. In an example, alloys optimized for mechanical strength have the highest resistance. Stainless steel (resistance approximately 27 times copper) is also employed for housings when requirements justify the cost (e.g., corrosion and chemical resistance requirements). Copper is rarely utilized for housings due to cost and low mechanical strength. The damping due to the housing varies in proportion to the respective resistivity of copper, brass, and stainless steel. Higher resistivity causes higher damping.
However, when a sensor is mounted, damping due to mounting and other surrounding metal may exceed the other sources of damping (e.g., coil resistance, housing, etc.). This effect intensifies as the mounting surface approaches a forward-most edge of the sensor's metallic housing, corresponding to the face of the sensor for shielded sensors and the rear of the plastic front end-cap for unshielded sensors.
“Shielded” sensors have metallic housings extending to the front face and may be mounted with the sensor's face up to and/or flush with the front mounting surface. “Unshielded” sensors have a metallic housing typically extending to within the range of 10 mm to 20 mm of the front sensor surface for a 30 mm tubular sensor, for example, where 30 mm denotes a size of a hole for mounting and not an actual outer diameter of the sensor.
Unshielded sensors exhibit greater sensitivity because (1) the magnetic field emanating from the sides of the sensor results in a greater magnetic flux density farther from the sensor face than for shielded sensors, and (2) unshielded sensors can be mounted with the mounting plane farther from the sensors' face, thus reducing the undesired damping. However, even in unshielded sensors, stray flux from the rear of the pot core interacts with the metallic housing.
One technique, to reduce undesired damping due to the housing, is to place a copper band, internal to the sensor and around the pot core, to shield the pot core from the housing. The current generated in the band reduces an amount of magnetic flux that reaches the brass or stainless steel housing. The damping is not reduced to zero and additional clearance may be necessary between the housing and the pot core to accommodate the band. Also, a copper disk or ring placed behind the pot core will similarly reduce stray flux from the rear.